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Full text of "Handbook Of Chemical Engineering - I"

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POWER GENERATION AND TRANSMISSION                   13
should be at least 28 in. from the grate: much more than this if soft coal is used. The bridge wall should come within 12 in. of the shell. The ashpit should be watertight and 1 in. below the firing-floor level, if ashes are removed by hand. Such boilers should be supported so as to pitch slightly down toward the rear. Pave the combustion chamber behind the bridge wall. Return of the gases rearward over the top of the boiler is unjustifiable: cover the top with brick or asbestos. Keep the side walls far enough apart for a suitably wide grate. Provide a large cleanout door through the setting wall at the rear.
Waste-heat Boilers.These include boilers using waste gases from beehive coke ovens, metallurgical furnaces, brick and cement kilns, etc. They are rarely worth installing unless the average temperature of the gases is 1,200F. or more. If it is 2,500F., the boilers may be of standard proportions, excepting for the grate and furnace. With lower temperature, more heating surface must be used per horsepower than is normal, and the gas velocities must be increased. This last requirement implies high (usually mechanical) draft. The draft at the primary furnace must not be impaired. The evaporation per square foot of heating surface may fall to 2 Ib. or so. The efficiency may exceed that of a boiler burning primary fuel, as there are no combustion losses to be considered. Dust and tar deposits may give trouble, and cleanout doors should be provided liberally. Settling chambers are often necessary. Gases containing sulphur are corrosive in the presence of moisture. Auxiliary grates, burning primary fuel, are sometimes used to steady conditions. If W Ib. of gas are available per hour at TF.j and if the amount of heating surface is sufficient to reduce the gas temperature to tF., the horsepower derived is W(T)s-i-33,479, where s = mean specific heat of gases.
AVERAGE WORKING TEMPERATURES OF WASTE GASES
(Babcock & Wilcox)
TEMPERATURE FAHREN-PROCESS                                                                                                  HEIT
Brick kilns.............................................      2,000-2,300
Zinc furnaces...........................................      2,000-2,300
Copper matte reverberatories.............................      2,000-2,200
(    Beehive coke ovens......................................      1,800-2,000
Cement 'kilns..........................................        1,200- 1,600
Nickel refining furnaces..................................      1,500-1,750
Open-hearth steel furnaces..............................        1,100-1,400
Sizes and Proportions.The heating surface (HS) of a boiler is that portion of the metal surface one side of which is in contact with hot gas and having water or steam on the other side. It is measured on the fire side. For n tubes, d in. diameter and I ft. long, the tube HS is irdln -r 12 sq. ft. Horizontal tubular boilers contain additional PIS in the shell (about % -rrDl -* 12 and heads (about Tr/288 (% D2 - ndz), where D = shell diameter, in. From 8 to 12 sq. ft. of PIS may be allowed per rated horsepower. A boiler horsepower is the equivalent of the evaporation of 34J^ Ib. of water per hour at atmospheric pressure from a feed temperature of 212F., or 34.5 X 970.4 = 33,479 B.t.u. per hour. At 10 sq. ft. HS per horsepower, the rate of transmission is 3,347.9 B.t.u. per -square foot per hour. The rate of evaporation, RE, is the weight of water evaporated per square foot of HS per hour. At normal output, it is ebtween 3 and 3M- Much higher values are possible if the grate area and draft are adequate.